Semiconductor Power Module Comprising Graphene

ABSTRACT

The invention is concerned with a semiconductor power module comprising an electrically and thermally conductive base plate ( 14 ) and a semiconductor chip ( 12 ) and where a first layer of graphene ( 32 ) is placed between the semiconductor chip ( 12 ) and the base plate ( 14 ) in electrical and thermal contact with a first side the base plate ( 14 ). Thereby the cooling of the semiconductor power module is improved.

FIELD OF INVENTION

The present invention relates to a semiconductor power module.

BACKGROUND

In high power applications such as in power transmission anddistribution systems, semiconductor components, often in in the form ofsemiconductor chips or dies, are used for a variety of purposes such asconverting between direct current and alternating current and formingcircuit breakers.

Due to the high voltage and current levels used, the number of chipsneeded is high. It is therefore common to place several such chips in asemiconductor power module, where the chips in the module may beconnected in parallel and/or in series.

Examples of semiconductor power modules can be found in EP 2544229, WO2012/107482, U.S. Pat. Nos. 6,426,561 and 9,099,567.

The current levels in such applications may thus be high. Thesemiconductor components may in some cases also be switched frequently.All in all, this may lead to a high power dissipation in the components.

Cooling is therefore an important aspect of a semiconductor powermodule.

SUMMARY OF THE INVENTION

One object of the present invention is therefore to improve the coolingof semiconductor chips in semiconductor power modules.

This object is achieved through a semiconductor power module comprisingan electrically and thermally conductive base plate and a semiconductorchip, wherein there is a first layer of graphene between thesemiconductor chip and the base plate in electrical and thermal contactwith a first side of the base plate.

As graphene has excellent thermal conductivity, the cooling of thesemiconductor chip is improved.

According to a first variation the semiconductor power module furthercomprises a second layer of graphene on a second, opposite side of thebase plate, which aids in improving the cooling.

The thickness of at least the first layer of graphene may be in therange 1-10 nm and preferably in the range 2-4 nm.

According to another variation there may be a first metallic layer withlow melting point between the chip and first layer of graphene, whichhas the advantage of improving the reliability of entering ashort-circuit failure mode.

Moreover, the first metallic layer may comprise a metal or metal alloylayer, which may be of aluminum and/or silver. The alloy may moreparticularly be an aluminium silver alloy or an aluminium siliconcarbide alloy.

According to another variation, the first metallic layer may be providedon a first side of the chip and a second metallic layer with low meltingpoint may be provided on a second, opposite side of the chip in order tofurther enhance the reliability of entering of the short-circuit failuremode.

The melting point of the first and second metallic layers may be in therange 500-700° C.

According to a further variation the semiconductor power module maycomprise a top electrode and an upper base plate on top of the secondmetallic layer. In this case the base plate may form a bottom electrode.

The semiconductor chip may with advantage be a silicon carbide chip. Thebase plate may in turn be of molybdenum.

According to another variation, the semiconductor power module maycomprise a number of additional semiconductor chips, each connected tothe base plate via the first layer of graphene.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be described with referencebeing made to the accompanying drawings, where

FIG. 1 schematically shows a semiconductor power module comprising anumber of submodules,

FIG. 2 schematically shows a submodule comprising a base plate and anumber of semiconductor chips, and

FIG. 3 schematically shows a part of the submodule provided in relationto one chip.

DETAILED DESCRIPTION OF THE INVENTION

The present invention concerns semiconductor power modules forsemiconductor chips or dies for instance for use in high voltageapplications, such as in power transmission or distribution systems.

In such modules a number of semiconductor chips are provided in astructure that connects the chips either in parallel or in series.

In the field of semiconductor power modules the wide bandgapsemiconductor, such as silicon carbide (SiC) semiconductor, is nowbelieved to be the favorite candidate for a diverse range of powerelectronics applications in energy transmission and distributionsystems. Hence, it offers potential replacement to leading horse silicon(Si) material counterpart used in a variety of semiconductor devices,such as Metal Oxide Semiconductor Field Effect Transistors (MOSFETs),Insulated-Gate Bipolar Transistors (IGBTs), Integrated Gate-CommutatedThyristors (IGCTs) etc., primarily because of higher field strength(i.e., 10× than that of Si), wider band gap (3× than that of Si), higherthermal conductivity (3× than that of Si), and higher carrier velocity(2× than that of Si). Earlier power electronic building blocks utilizedin a variety of power converter topologies in energy transmission anddistribution systems were mostly limited to Si based power switches suchas (IGBTs) and (IGCTs). From a power system design point of view,semiconductor chips that enable reduced power losses, provide higherpower density, facilitate compact converter design, lower environmentalimpact and simultaneously bring lower overall system cost will beconsidered key technological booster for very high power applications.With these key performance indicators in mind, finding a good solutionin terms of chosen semiconductor technology that better fits therequirements given by the specific topologies and standardization ofconverter building block design presents one advancement in productdevelopment.

Recent trends in high power electronics have induced increasing demandson improving the efficiency of power converter on one side andsimultaneously reducing the footprint of the power electronic buildingblocks on the other hand. A better thermal management is therefore apre-requisite to achieve this objective especially when several powerchips with high blocking voltages are sitting in parallel to achievehigh current capability. This has further resulted in higher powerdissipation densities for the IGBT die as well as the power module, dueto denser packing of the die itself. On the other side, increase in theswitching frequency and voltage ratings of IGBTs also result in higherpower dissipation at the die level. A relaxed cooling requirement undercontinuous operation is therefore critical. A substandard coolingcapability may limit the device performance.

In operation such semiconductor chips may thus carry high current andmay at times be switched frequently and therefore generate a lot ofheat. This makes cooling important.

Furthermore, for the power modules used in modular multilevel converter(MMC) for instance in High Voltage Direct Current (HVDC) applications,the switching device (i.e., Si-IGBTs) should carry the load currentunder short circuit failure mode (SCFM) condition for some period oftime until the next maintenance. Accordingly, a very stable shortcircuit condition through the failed module must be formed andguaranteed until the system is serviced. This SCFM mode has an importantconsequence on the power module design since a single failed chip andits contact system should take up the whole module current of up to atleast 1500 A (phase-rms), where rms denotes root mean square.

In a power module the reliable short circuit is typically obtainedthrough the use of a metallic layer with low melting point, such as anAluminum Silver (Al—Si) alloy, placed above the chip. When there is ashort-circuit, this metallic layer melts and forms an alloy with thesilicon of the chip due to high induced energy for short period of time.Hence the layer forms a low resistance stable alloy with the Si die.Note that metals like silver and aluminum are generally preferred asthey form low melting eutectic alloys with silicon die underneath.

However, if an SiC semiconductor chip is used in the semiconductor powermodule, the operation of the switching device under SCFM condition inMMC may become unpredictable since the melting point of the SiC materialis higher than that of the Si-counterpart and hence stable alloyformation that short-circuits the collector-emitter may be questionable.

A reliable short-circuit is thus also important.

Aspects of the invention are directed towards the above-mentioned twoareas namely the area of improving cooling and improving the reliabilityof the SFCM mode, especially in relation to SiC chips.

The semiconductor power module may comprise a power module comprisingone or more submodules, where such submodules may be electricallyconnected in series and/or in parallel with each other. A semiconductorpower module to comprising six submodules 11 is shown in FIG. 1. Itshould be realized that a power module may comprise more or fewersubmodules than the ones shown.

An exemplifying submodule 11 of the semiconductor power module to isschematically shown in FIG. 2.

In the submodule 11 there is a base plate 14, which is made of a metalsuch as Molybdenum and enclosed by an inner frame 30 and an outer frame31, which frames may be made of a plastic material. On this base plate14 a number of semiconductor chips 12 are placed. In the example thereare six chips provided in two rows. In the figure only one row withthree chips is shown. Moreover, a first side of the base plate 14 facingthe chips 12 is covered by a first material section 18, where a secondopposite side of the base plate 14 is covered by a second materialsection 20. Moreover each chip 12 is connected to an upper plate 16 viaa corresponding third material section 22, a current bypass element 24,typically made of copper, and a spring 26. Here the upper plate 16,which may also be of copper, presses down on an upper part of thecurrent bypass element 24 with is biased via the spring 26 against alower part of the same in order to ensure a galvanic contact between thechip 12 and upper plate 16 via the third material section 22 as well asbetween the chip 12 and base plate 14 via the first material section 18.

It may here be mentioned that the structure shown in FIG. 2 is merelyone way of realizing a submodule and that the chips may be electricallyconnected in series or parallel with each other.

FIG. 3 shows relevant parts of the structure of the submodule 11 in FIG.2 for one of the semiconductor chips 12 in more detail.

The first material section 18 below the chip 12 in this case comprises afirst layer of graphene 32 electrically and thermally in contact withthe first side of the base plate 14, which graphene layer 32 is withadvantage 1-10 nm and preferably 2-4 nm thick. The first materialsection 18 furthermore comprises, on top of the first graphene layer 32and below the chip 12, a first metallic layer 34 with a low meltingpoint, which melting point may be in the range 500-700° C. This layer34, which is placed on top of the graphene layer 32, may be a layer thatis based on aluminum or silver or it may be an alloy based on aluminumor silver, such as an aluminum silver alloy or an aluminum siliconcarbide alloy. This layer may thus be Al or Al—Ag alloy or AlSiC or anyother suitable metal alloy having high thermal and electricalconductivity and with low melting point (e.g., <700 or 650). The thermalconductivity of the first metallic layer 34 may be above 1.5 W/cm-K andwith advantage in the range of 1.5-4.5 W/cm-K, where in case Al is usedthe thermal conductivity may be 2.0 W/cm-K, in case Ag is used it may be4.2 W/cm-K and in case AlSiC is used it may be in the range 1.8-2.1.

The second material section 20 in turn comprises a second layer ofgraphene in electrical and thermal contact with the second side of thebase plate 14, which layer is with advantage also 1-10 nm and preferably2-4 nm thick.

Moreover, the third material section 22 comprises a second metalliclayer 36 with low melting point, such as aluminum or an aluminum silveralloy, on top of the chip 12, an upper base plate in the form of a layer38 of high melting point and good conductivity, for instance molybdenum,on top of the layer 36 and a chip terminal contact material 40 on top oflayer 36, where the chip terminal contact 40 may be an emitter sidecontact of for instance copper. Thereby the base plate 14 may also forma collector side contact. It should here be realized that there areother ways in which the third material section 22 may be realized. Thethermal conductivity of the second metallic layer 36 may be above 1.5W/cm-K and with advantage in the range of 1.5-4.5 W/cm-K, where in caseAl is used the thermal conductivity may be 2.0 W/cm-K, in case Ag isused it may be 4.2 W/cm-K and in case AlSiC is used it may be in therange 1.8-2.1.

Finally the semiconductor chip is in this embodiment a silicone carbidechip, which has a high meting point in the range of 2400 K.

Power dissipated in a SiC chip may be high. The two layers of graphene20 and 32, which have excellent thermal conductivity properties, areprovided in order to funnel away such heat and thus improve the thermalmanagement in normal continuous operation of the chip 12.

Thin layers 20 and 32 of graphene are thus inserted around the Mo bottomplate 14, which is thereby sandwiched between the graphene layers 20 and32. A comparative assessment of 2D (two dimensional) graphene with othertraditional semiconductor materials is presented in table I below, Ascan be seen, graphene has superior electrical and thermal propertiescompared to other semiconductors such as silicone (Si), gallium arsenide(GaAs), indium phosphide (InP), silicon carbide (SiC) and galliumnitride (GaN).

TABLE I Property Si GaAs InP SiC GaN Graphene Electron 1400 8500 5400950 1000 >15000 mobility (cm³/V · s) Hole 450 400 200 120 30 >15000mobility (cm³/V · s) Electron 1.0 1.3 2.3 2.0 2.5 4-5 velocity (×10⁷cm/s) Thermal 1.5 0.55 0.68 4.9 1.3-1.5 48-53 conductivity (W/cm-K)Young's 140 85 61 450 295 1000 modulus (GPa)

These graphene properties are summarized as follows:

-   -   Extremely thin material sheet, while still being very strong (5        times stronger than steel and much lighter),    -   Graphene is semi-metal or zero bandgap semiconductor,    -   Superb heat conductor (even far better than SiC, copper, diamond        etc.),    -   More transparent than ITO (i.e., Indium tin oxide used in        photovoltaics, touch screen displays) over much large spectral        band, and    -   Easy growth/manufacturability on metal or semiconductor        substrate.

Growth mechanisms through variety of techniques are well-established.

The purpose of the first and second metallic layers 34 and 36 with lowmelting point is to obtain a short-circuit failure mode. There the boththe first and the second metallic layer 34 and 36 with low melting pointwill melt during a short-circuit and create an electrically conductivealloy with the semiconductor chip 12.

As there are two such metallic layers on both sides of the chip, theshort-circuit is more reliable than if there is only one.

An SiC chip is typically thinner than a conventional Si-IGBT die.Similarly, large difference in the thermal expansion coefficient (CTE)of upper/lower metals 36 and 34 (e.g., CTE of Al, Ag and AlSiC is 19,23, 7.5, respectively) with SiC chip (i.e., CTE of 2.7) combined withamount of the heat generated as a result of high current densities andhigh temperature may damage the chip surface and hence induce the shortcircuit condition. Note that the melting point of AlSiC (with 63% volumeof SiC) is 557-613° C. and thermal conductivity stays around 1.7-2.1W/cm-K.

Two new features have thus been introduced.

1: A new layer 34 of suitable metal and/or metal alloy with low meltingpoint and high thermal and electrical conductivity underneath the SiCchip. This will help to attain stable short circuit formation (SCFM)once the SiC chip is failed.2: One or two thin layers 20 and 32 of graphene material around thebottom Mo plate 14 for improved cooling and better thermal management ofthe power module.

As the first layer of graphene has a high electrical conductivity andhigh thermal conductivity it is clear that the reliability of enteringthe SCFM mode is not jeopardized. The improved cooling is thereforeobtained while at the same retaining the reliability with which the SCFMmode is entered upon short-circuits.

In an analogous manner the first metallic layer has a thermalconductivity that is high enough to aid the graphene layers in thecooling of the chip while its ability to safely enter the SCFM mode isunaffected.

As mentioned above the chip is with advantage an SiC chip. It shouldhowever be realized that the chip may just as well be another type ofchip of any of the above-mentioned semiconductor materials, such as forinstance an Si chip.

Moreover the semiconductor chip is with advantage realized as a switchsuch as a transistor like an IGBT or MOSFET transistor or a thyristorsuch as an IGCT.

From the foregoing discussion it is evident that the present inventioncan be varied in a multitude of ways.

It shall consequently be realized that the present invention is only tobe limited by the following claims.

1. A semiconductor power module comprising: an electrically and thermally conductive base plate, a semiconductor chip, a first layer of graphene between the semiconductor chip and the base plate, where the layer is in electrical and thermal contact with a first side of the base plate, a first metallic layer with low melting point on the first side of the chip between the chip and first layer of graphene, a second metallic layer with low melting point on a second, opposite side of the chip, said metallic layers being provided for obtaining a short-circuit failure mode of the semiconductor chip creating an electrically conducting alloy with the semiconductor chip, a top electrode, and an upper base plate on top of the second metallic layer.
 2. The semiconductor power module according to claim 1, further comprising a second layer of graphene on a second, opposite side of the base plate.
 3. The semiconductor power module according to claim 1, wherein the thickness of at least the first layer of graphene is in the range 1-10 nm.
 4. The semiconductor power module according to claim 1, wherein the melting point is in the range 500-700° C.
 5. The semiconductor power module according to claim 4, wherein the first metallic layer 34 has a thermal conductivity that is above 1.5 W/cm-K.
 6. The semiconductor power module according to claim 5, wherein the thermal conductivity is in the range of 1.5-4.5 W/cm-K.
 7. The semiconductor power module according to claim 1, wherein the first metallic layer includes a metal or metal alloy layer.
 8. The semiconductor power module according to claim 7, wherein the layer includes aluminum and/or silver.
 9. The semiconductor power module according to claim 7, wherein the alloy is an aluminum silver alloy or an aluminum silicon carbide alloy.
 10. The semiconductor power module according to claim 1, wherein the semiconductor chip is a silicon carbide chip.
 11. The semiconductor power module according to claim 1, wherein the base plate is of molybdenum.
 12. The semiconductor power module according to claim 1, further comprising a number of additional semiconductor chips, each connected to the base plate via said first layer of graphene.
 13. The semiconductor power module according to claim 1, wherein the thickness of at least the first layer of graphene is in the range 2-4 nm.
 14. The semiconductor power module according to claim 2, wherein the thickness of at least the first layer of graphene is in the range 1-10 nm.
 15. The semiconductor power module according to claim 2, wherein the melting point is in the range 500-700° C.
 16. The semiconductor power module according to claim 2, wherein the first metallic layer includes a metal or metal alloy layer.
 17. The semiconductor power module according to claim 8, wherein the alloy is an aluminum silver alloy or an aluminum silicon carbide alloy.
 18. The semiconductor power module according to claim 2, wherein the semiconductor chip is a silicon carbide chip.
 19. The semiconductor power module according to claim 2, wherein the base plate is of molybdenum. 